This project investigates biobehavioral development through comparative longitudinal study of rhesus and capuchin monkeys, with special emphasis on characterizing individual differences in behavioral and physiological responses to mild environmental challenges and on determining long-term developmental consequences for individuals reared in different physical and social environments. During FY91 several studies provided new insights regarding both genetic and environmental factors influencing such responses in a wide range of subjects. Repeated measures of physical, physiological, and behavioral development over the first 5 months of life were obtained from a large cohort of nursery-reared monkey infants and a mother-reared comparison group as the initial phase in a long-term prospective study. Some of the infants in both cohorts were offspring of selectively bred parents with unusually low or high CSF 5-HIAA levels. Continued longitudinal study of similarly reared older monkeys revealed a strong association between high levels of aggression during adolesence and low levels of CSF 5-HIAA The finding of a strong association between aggressive behavior and 5-HIAA levels was replicated on another sample of adolescent male rhesus monkeys living in wild troops. Data from a second study of adolescent males living in wild troops detailed the relationship between the males' timing and pattern of natal troop emigration and their pattern of cardiac and hormonal response to capture and brief confinement. A significant correlation between the heartrate patterns displayed by these males following emigration and those of their biological mothers back in their natal troop was also disclosed. In addition, behavioral data collected from infants prior to, during, and following natural short-term maternal separations during breeding activity in these same wild troops revealed a strong predictive relationship between severity of separation reaction and behavioral, adrenocortical, cardiac, and immunological response to subsequent capture and brief confinement, thus replicating in a monkey population robust findings previously demonstrated under laboratory conditions. Finally, a major longitudinal study of individual differences in biobehavioral response to novelty and challenge in capuchin monkey infants growing up in different physical and social settings was begun.